The many engineering challenges that face the replacement of wires used in wearable electronic devices with wireless links continue to be barriers to their deployment. First and foremost is the fact that any so-called “wearable” electronic device must be by definition lightweight and small in size. This restriction applies to the power source, meaning very little current must be used by these devices if the available power is to last more than a short period of time. The excessive power demands of existing wireless systems have thus far limited their deployment to applications with larger power sources, where the additional current drain represented by the wireless link is not significant.
There are emerging wireless systems (currently in the development stage at the writing of this patent) attempting to address this problem, however they utilize existing conventional transceiver architectures—lower average power consumption is achieved with shortened duty cycles and quiescent sleep modes. These techniques have the additional unwanted effect of reducing the average data rates for the link. What is required are novel transceiver architectures which can operate continuously using very little power.
Any such system will have to address the issue of radio channel path loss between the Controller and Terminal. Since all RF signals in the link are generated by the Controller, a double path loss (Controller to Terminal and back to Controller again) would be incurred by the signals used in the link. There are two possible solutions to this dilemma, first, the signals transmitted by the Controller may be amplified sufficiently to overcome the increased path loss. This is counter to the low-power-consumption design requirement described above for wearable systems. The second solution is to build a system which utilizes “processing gain” to facilitate reception of very low power signals [1]. All references referred to in square brackets are listed at the end of this patent disclosure and incorporated herein by reference.
Processing gain may be provided by Direct Sequence Spread Spectrum” (DSSS) techniques, which utilize digital pseudo-noise sequences with a bandwidth many times that of the data signal [2]. Such systems are sub-optimal for low power operation, however, due to the complexity of signal acquisition, and synchronization schemes which must be implemented in the receiver. DSSS systems have substantial quiescent power drain, even when no data is being sent.
Processing gain may also be provided by using chirps rather than digital pseudo-noise sequences, and is sometimes called “Chirp Spread Spectrum” or (CSS) [3]. This modulation method has the advantage of requiring, no synchronization and thus requires little or no quiescent power drain. CSS can also be implemented completely in analog hardware (i.e. with no DSP), thus negating the tradeoff between bandwidth and power consumption inherent in digital systems. Very large processing gains (>10,000) are practical in CSS systems, since there is no synchronization load. Signals may also overlap in time to further increase the data rate [3].
Chirps signals have been used in a variety of systems including RADAR systems [4], two way wireless communication systems [5-8] and U.S. Pat. Nos. 6,466,609; 6,453,200; 6,940,893; 6,788,204; 6,144,288, and also in Australian patent no. 720,596.
There are known wireless communication systems which allow for asymmetry in the link transceivers, as for example in RF ID tag technology. In these systems (examples are U.S. Pat. Nos. 6,796,508, 6,791,489, 6,784,813 and 6,784,787—there, are many more) the limited function side of the link is usually reduced to complete passivity (i.e. unpowered), relying on excitation from the full-function side to initiate operation.
There remains an engineering challenge to provide full two way communication over short distances with low power consumption.